memcache
A memcached client library.
https://github.com/dterei/memcache-hs#readme
| LTS Haskell 24.64: | 0.3.0.2 |
| Stackage Nightly 2026-10-20: | 0.4.2.0 |
| Latest on Hackage: | 0.4.2.0 |
memcache-0.4.2.0@sha256:1aa271847ea66ab931d625fb067f1366cc22c03dc10c15e5c7caa4eaa4e62d9e,4574Module documentation for 0.4.2.0
memcache: Haskell Memcached Client
A client library for a memcached cluster.
It supports the Meta Text memcached protocol and username/password authentication. Keys containing whitespace or control bytes are sent using its base64 binary-key mode. The encoded key must still fit within 250 bytes. It supports connecting to a single, or a cluster of memcached servers. When connecting to a cluster, consistent hashing is used for routing requests to the appropriate server.
This library requires memcached 1.6.0 or newer, the first release with the
meta commands. Against older servers every operation is rejected with ERROR.
Authentication uses a memcached auth file with -Y authfile, an experimental
ASCII-protocol auth mode added in memcached 1.5.15; the old binary protocol
-S SASL mode is not supported. Enabling -Y also disables binary and UDP
protocols on the server. getMany uses quiet misses, so a server connection
failure can be indistinguishable from a cache miss.
The client operations, including getMany, use direct Meta Text encoders and
the Response type; the former request and response wrapper types are no longer
provided.
Usage
Add memcache to your package’s build-depends. Everything most programs need
is exported from Database.Memcache.Client:
{-# LANGUAGE OverloadedStrings #-}
module Main (main) where
import qualified Database.Memcache.Client as M
main :: IO ()
main = do
-- Connect to memcached on 127.0.0.1:11211 with the default options.
mc <- M.newClient [M.def] M.def
-- Store a value with flags 0 that never expires, then read it back along
-- with its flags and CAS version.
_ <- M.set mc "greeting" "hello" 0 0
hit <- M.get mc "greeting"
case hit of
Nothing -> putStrLn "miss"
Just (value, _flags, version) -> do
print value
-- Only overwrite the value if it hasn't changed since we read it.
updated <- M.cas mc "greeting" "hello again" 0 0 version
putStrLn (maybe "changed by someone else" (const "updated") updated)
-- Create the counter as 1 if it is missing, otherwise add 1 to it.
visits <- M.increment mc "visits" 1 1 0 0
print (fst <$> visits)
M.quit mc
Keys and values are strict ByteStrings. Misses are returned as Nothing,
while unexpected failures are thrown as a MemcacheError or a network
IOException. Operations that take a CAS Version skip the check when it is
0. increment and decrement create a missing key with the given initial
value, unless the expiration is maxBound. The
Database.Memcache.Client
documentation covers every operation, along with the timeouts, retries and
failover configured through Options.
Servers and authentication
newClient takes one ServerSpec per server, and routes each key to one of
them. Credentials are set per server:
mc <- M.newClient
[ M.ServerSpec "cache1.internal" "11211" (M.Auth "user" "secret")
, M.ServerSpec "cache2.internal" "11211" (M.Auth "user" "secret")
]
M.def
parseServerSpec, from Database.Memcache.Types, reads a server from a
memcached://[user:password@]host[:port] URI instead. The host defaults to
127.0.0.1 and the port to 11211:
import Database.Memcache.Types (parseServerSpec)
spec <- either fail pure (parseServerSpec "memcached://user:[email protected]:11211")
mc <- M.newClient [spec] M.def
The server must be started with -Y <authfile>, where the auth file holds one
username:password line per user. This library connects without TLS, so
credentials and all data are sent unencrypted; only rely on them within a
trusted network.
Amazon ElastiCache
Database.Memcache.ElastiCacheClient creates a client from an ElastiCache
configuration endpoint (the address containing .cfg). It discovers the
cluster’s nodes, then refreshes the node list in a background thread on the
given interval, in microseconds as for threadDelay:
import qualified Database.Memcache.ElastiCacheClient as EC
endpoint <- either fail pure $
EC.parseConfigurationEndpoint "memcached://mycluster.abc123.cfg.use1.cache.amazonaws.com:11211"
mc <- EC.newClient M.def (60 * 1000000) endpoint
Licensing
This library is BSD-licensed.
Tools
This library also includes a few tools for manipulating and experimenting with memcached servers.
OpGen– A load generator for memcached. Doesn’t collect timing statistics, other tools like mutilate already do that very well. This tool is useful in conjunction with mutilate.Loader– A tool to load random data of a certain size into a memcached server. Useful for priming a server for testing.
Architecture Notes
We’re relying on Data.Pool for thread safety right now. Each server has its
own pool, and grabbing a connection from it (withResource) blocks other
requests using that connection until the operation completes. getMany is the
exception: it pipelines quiet Meta Text requests over one pooled connection.
By default each server’s pool holds a single connection (soNumResources = 1),
so concurrent operations against the same server wait for each other. Raising
soNumResources in the ServerOptions set through optsServerOptions allows
concurrent operations without requiring a separate pool implementation.
Either way, a pool is fine for now.
Other clients
Get involved!
We are happy to receive bug reports, fixes, documentation enhancements, and other improvements.
Please report bugs via the github issue tracker, and security issues as described in the security policy.
Master git repository:
git clone https://github.com/dterei/memcache-hs.git
Development
The package builds with Stack or cabal, on GHC 9.2 or newer:
stack build
stack test
cabal build
cabal test all
stack.yaml uses Stackage LTS 22.14 (GHC 9.6). The oldest and newest snapshots
CI tests against have their own files, for example
stack --stack-yaml stack-lts-20.26.yaml test for GHC 9.2.
There are two test suites:
specholds the unit and property tests, which run against an in-process mock server and need nothing installed.integrationstarts its ownmemcachedservers on free local ports and tests against them, so it needs memcached 1.6 or newer on yourPATH. Without it, the suite reports a single pending test instead of failing. memcached refuses to run as root without-u, so run the suite as an ordinary user.
CI lints with HLint 3.10, so please
check that hlint . prints No hints before opening a pull request. For any
user-visible change, add a line to CHANGELOG.md under an
Unreleased heading at the top, adding the heading if it isn’t there.
CI runs both build tools on every pull request. The CI workflow builds and
tests with Stack against each stack*.yaml snapshot, and runs HLint and
weeder. The Cabal workflow builds and tests with cabal on GHC 9.2 to 9.14,
builds the library against the oldest dependency versions its bounds allow,
and builds and tests the source distribution; it also runs every Monday, so a
new dependency release that breaks the build shows up without a push. Both
install memcached, so the integration suite runs in full.
Authors
This library is written and maintained by David Terei ([email protected]).
Contributions have been made by the following great people:
- Alfredo Di Napoli ([email protected])
- Amit Levy
- Chris Martin
- drlkf
- Joel McCracken
- Steven Leiva